1. Technical Field
The present invention relates in general to a current mode pulse width modulation (PWM) technique for brushless motors and in particular to a system and method for controlling a brushless motor by increasing or decreasing motor speed respectively according to the positive polarity or negative polarity of a pulse width modulation (PWM) control signal wherein the PWM signal varies according to an error signal generated and derived between the difference of a desired motor speed and a current motor speed.
2. Description of the Related Art
Various systems and methods exist for controlling the speed of a brushless motor. One conventional prior art system and method involves using a pulse width modulation (PWM) signal to directly control the power switches to the brushless motor. For example, a three phase motor would have three upper switches and three lower switches to control the brushless motor. A pair of upper and lower switches is used to control each phase of the brushless motor. In this prior art system and method, the PWM signal controls the brushless motor in a manner such that the six switches are either on to excite the motor or off to turn off or slow down the motor speed. Thus, motor speed is increased by having the PWM signal activate the power switches to drive the motor, and motor speed is decreased by having the PWM signal deactivate the power switches to shut off the motor. Therefore, the motor is continuously and repeatedly switched between its on and off states resulting in full power excitation in one direction or coast condition.
Tape drives that use brushless motors to drive reels of tape for computer systems are in existence in the computer field. One type of tape drive, such as Linear Tape Open (LTO), involves two reels of tape wherein one of the two reels is a supply reel and the other of the two reels is a take-up reel. The supply reel is driven by a supply reel motor while the take-up reel is driven by a take-up reel motor. Control algorithms are used to control the speed of these two motors. For these types of tape drives or tape transports, the position and velocity of the tape need to be controlled. The control algorithms require the determination of the position and velocity of the tape in order to properly control the speed of the reel motors. The two tape reels, tape, and two motors that drive the reels are considered a plant. The plant receives multiple inputs and provides multiple outputs. For example, inputs to the plant are continuous currents to the motors for controlling the motors. The outputs from the plant are tape velocity, tape tension, and tape position.
These types of tape drives require that a desired motor speed for the tape drive be compared with the actual motor speed in order to properly control the tape position and velocity. If the desired motor speed is not equal to the actual motor speed, then the motor needs to be driven from the actual motor speed to the desired motor speed. It is therefore advantageous and desirable to provide a system and method for controlling the speed of a brushless motor according to and based on the difference between a desired motor speed and an actual motor speed. It is also advantageous and desirable to provide a system and method for controlling the speed of a brushless motor that uses a PWM signal that does not require the continuous and repeated turning on and off of the motor but that allows the brushless motor to be continuously driven. It is further advantageous and desirable to provide a system and method for controlling the speed of a brushless motor that uses a PWM signal for controlling the speed of a brushless motor according to and based on the difference between a desired motor speed and an actual motor speed. It is still also advantageous and desirable to provide a system and method for controlling the speed of a brushless motor that is used in a tape servo system and method. It is still further advantageous and desirable to provide a system and method for controlling the speed of a brushless motor based on armature positions of the brushless motor.
It is therefore one object of the present invention to provide a system and method for controlling the speed of a brushless motor according to and based on the difference between a desired motor speed and an actual motor speed.
It is another object of the present invention to provide a system and method for controlling the speed of a brushless motor that uses a PWM signal that does not require the continuous and repeated turning on and off of the motor but that allows the brushless motor to be continuously driven.
It is a further object of the present invention to provide a system and method for controlling the speed of a brushless motor that uses a PWM signal for controlling the speed of a brushless motor according to and based on the difference between a desired motor speed and an actual motor speed.
It is still a further object of the present invention to provide a system and method for controlling the speed of a brushless motor that is used in a tape servo system and method.
It is still another object of the present invention to provide a system and method for controlling the speed of a brushless motor based on armature positions of the brushless motor.
The foregoing objects are achieved as is now described. A current mode pulse width modulation (PWM) technique for brushless motors. The present system and method controls a brushless motor by increasing or decreasing motor speed respectively according to the polarity and value of a pulse width modulation (PWM) control signal. The PWM signal varies according to an error signal generated in proportion to the difference between a desired motor current and an actual motor current. A motor current sensor is used to sense an actual motor current being applied to the motor. A current comparator is used to compare the current desired reference signal and the actual current signal and to generate an error signal. A pulse width modulator is used to convert the error signal to a pulse-width modulated error signal. Armature position sensors are used for sensing armature positions of the motor. A commutator circuit receives the pulse-width modulated error signal and the armature positions of the motor. Power switches are coupled to outputs of the commutator and also coupled to the motor. The commutator controls activation and deactivation of the respective power switches based on the corresponding pulse-width modulated error signal to control the motor at a desired motor current that corresponds to the desired reference current signal. The commutator controls the respective power switches to drive the motor in either a more positive, a more negative direction, or in the same manner to respectively increase, decrease, or maintain the motor speed when the error signal correspondingly has a generally positive, negative, or zero value.
The above as well as additional objects, features, and advantages of the present invention will become apparent in the following detailed written description.